Rate responsive pacemakers have gained wide acceptance as providing a response which is adaptable to a patient's physiological needs. Normally, as a patient undertakes exercise or becomes involved in a situation which calls for an increased heart rate, the patient's normal feedback mechanisms provide for the increased rate, whereby more blood is delivered by the heart. However, such rate feedback mechanisms may be impaired. Further, even if the heart's natural pacemaker adapts to changing demand, a single chamber pacemaker would be unresponsive to desired rate changes. The rate responsive pacemaker provides a substantially physiologic pacing rate by sensing one or more patient parameters, and correlating desired rate with such one or more parameters to mimic the natural feedback mechanisms. Rate responsive pacemakers can be single chamber, e.g., VVIR; dual chamber, e.g., DDDR; or even multiple chamber.
The first commercially available form of single chamber rate responsive pacemaker was the QT driven rate responsive pacemaker, marketed by the assignee of this invention. See U.S. Pat. No. 4,228,803, Rickards. Subsequently, other pacemakers utilizing other monitored variables have been introduced commercially, e.g., the Activitrax and other activity sensor-based models manufactured by Medtronic, Inc. Whatever the monitored parameter or parameters for indicating desired pacing rate, the general control of the rate responsive pacemaker is common and generic. A rate responsive pacemaker incorporates one or more sensors adapted to sense patients variables which continually indicate, or reflect the patient's cardiac output needs, and from which quantitative information can be derived. For example, in the QT pacemaker, the sensor is the same lead that delivers the pacing pulses. Following delivery of a pacing pulse, it senses the stimulus-evoked T-wave, and provides an electrical signal which can be compared in the time domain to the stimulus pulse, thereby providing the QT time interval information. Dual sensor rate responsive pacemakers use two sensors for monitoring two different parameters, the two parameter signals being combined to advantageously utilize the information from each. Thus, the assignee of this invention makes a dual sensor rate responsive pacemaker which uses both QT interval and activity counts to provide the rate information. Hereafter, the terms "rate responsive parameter", "rate parameter" and "sensor" refer to and incorporate either a single sensor, dual sensor or multiple sensor system.
The general structure of a rate responsive pacemaker also includes a means for carrying out an algorithm by which the pacemaker determines the indicated pacing rate on the basis of information delivered by the sensor. The function of the algorithm is to provide a correlation between the sensed rate parameter and pacing rate, so that the pacemaker is accurately responsive to the monitored information. Using the QT driven rate responsive pacemaker as an example, the algorithm determines, for the current pacing rate, the desired change in pacing rate per ms change in stimulus-T interval, which is defined as slope. In mathematical terms, the algorithm provides the function of rate vs. QT, over a range which is bounded at a lower rate limit (LRL) and an upper rate limit (URL). For an activity-based pacemaker, the algorithm provides the function of rate vs. activity count, for the range between LRL and URL. The algorithm is suitably stored as data in memory, from which the pacemaker can find on a cycle-to-cycle basis what the change in pacing rate should be for a sensed rate parameter change. While the algorithm is initially stored in the pacemaker at implant, or programmed at time of implant, it is known that the correlation between pacing rate and the rate parameter can change for any patient. For this reason, the pacemaker should have the capacity to test and adapt the rate response function, i.e., the algorithm, as a function of patient history. See U.S. Pat. No. 4,972,834, incorporated herein by reference, which provides for dynamic rate responsiveness by automatic adaptation of the algorithm. As set forth in the referenced patent, the slope of QT vs. pacing rate at LRL is measured each night, and the algorithm is adjusted based on this measurement. Likewise, an activity-based rate responsive pacemaker can undergo a similar daily test for the purpose of adapting the algorithm.
A problem with the daily test of the rate responsive algorithm, as provided in the referenced U.S. Pat. No. 4,972,834, is that the test can be upsetting, since it involves pacing at different rates in order to determine the patient's correlation of pacing rate and sensed parameter at one or more rates; such test pacing can be felt by the patient and be unsettling, especially if undertaken at night. Further, for many patients the pacemaker may normally be on standby at night and not be delivering pace pulses, such that the test involves overdrive pacing which takes additional power. Accordingly, it is desirable to provide the pacemaker with the capability of reducing the frequency of testing the algorithm, e.g., provide for longer intervals between tests when the history of the patient indicates that relatively little adjustment is required on an on-going basis.